Method for determining absolute reflectance of a material in the ultraviolet range

ABSTRACT

A method for determining a value of absolute reflectance of a material at .[.a predetermined.]. .Iadd.any .Iaddend.wavelength, in the ultraviolet range from its measured reflectance which includes system losses contributed by optics, illumination sources, detectors, etc. The method involves the measurement of reflectance from a known material such as single crystal silicon whose absolute reflectance is well known, dividing the measured value by the absolute value to obtain a system efficiency coefficient at the known wavelength and then, without changing the illumination or optics, measuring the reflectance of the unknown material and applying this coefficient to this measured value to obtain its absolute value.

BRIEF SUMMARY OF THE INVENTION

This invention relates to the determination of reflectance from amaterial and particularly to the determination of absolute reflectanceindependent of losses in an associated optical system.

Some materials are transparent with very little incident energy beingreflected from the surface and some materials are opaque and absorbnearly all incident energy and reflect very little. In many materialsthe .[.ratio of incident energy to reflected energy, or.].reflectance.[.,.]. varies according to the wavelength of the incidentradiation. For example, silicon as used in electronic microcircuits istransparent to infrared wavelengths, translucent between about onemicron and infrared, and opaque to ultraviolet radiation.

It is often necessary to determine the energy absorption of somematerial with unknown chemical contents. An accurate value forabsorption can easily be computed from a knowledge of the absolutereflectance from the unknown material since the incident energy can onlybe divided into absorption and reflectance. But a value for absolutereflectance is not readily obtainable since any measured value ofreflectance at some predetermined wavelength is contaminated by lossescontributed by the system optics, such as absorption of lenses,illumination sources, beamsplitters, gratings, detectors, etc., all ofwhich also vary with wavelengths.

The object of this invention is to determine the absolute reflectancevalue of a test material at a desired wavelength, from a measured valueof reflectance.

Briefly described, the method involves the steps of measuring thereflectance of a known material, such as single crystal silicon oraluminum specimen, at the desired wavelength, computing the value ofabsolute reflectance from .Iadd.the index of refraction and.Iaddend.absorption data available in .[.myriads of.]. handbooks, andthen dividing the absolute value .[.by.]. .Iadd.into .Iaddend.themeasured value to derive a .[.product of all optical systemcoefficients.]. .Iadd.system efficiency coefficient.Iaddend.. This valueof the .[.coefficients.]. .Iadd.coefficient .Iaddend.is stored. Anunknown material is then tested with the same unchanged optical systemand the same wavelength to obtain a measured reflectance value which,when .[.multiplied.]. .Iadd.divided .Iaddend.by the stored coefficient,yields the absolute reflectance value of the unknown material.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which illustrate the preferred embodiment of theinvention:

FIG. 1 is a schematic drawing illustrating a reflective microscope anddetector processing apparatus for ultraviolet examinations; and

FIG. 2 illustrate the steps for determining absolute reflectance of anunknown material.

DETAILED DESCRIPTION

FIG. 1 illustrates a typical microscope for measuring reflectance in theultraviolet range. Since the material used in the construction oftypical refractive lenses .[.chromatic or.]. is opaque to UV radiation,.[.only.]. reflective optical devices .[.may.]. .Iadd.should .Iaddend.beused. Hence, a UV source 10 with known output energy, such as adeuterium discharge tube, with output beam condensed by a small iris 12is reflected from a planar quartz beam splitter 14 into a reflectiveobjective lens 16 which focuses the UV beam onto a specimen 18. Theimage from the specimen 18 is magnified by the reflective objective and,after passing through the planar quartz beam splitter 14, is focusedinto a monochromator 20 where the image beam is passed through a narrowslit to isolate a narrow band of wavelengths before detection.

The detected image beam, after being converted by an analog-to-digitalconverter 22, is applied to a computer 24 having a memory 26 and aninput-output device 28, such as a keyboard terminal.

When the microscope system of FIG. 1 is used for measuring reflectances,all optical elements such as the beam splitter 14, the severalreflective surfaces of the objective lens 16 and the internal opticalelements in the monochromator 20, absorb energy from the incidentradiating beam of the source 10. Further, the absorbed energy varieswith variations in the incident wavelength.

A value of reflectance can easily be measured with the microscopesystem, but that is a measured reflectance, mR, which has little valuesince it includes the unknown system losses resulting from the energyabsorption of the optical elements. The type of reflectance of value isthe absolute reflectance, aR. the ratio of reflected energy to incidentenergy, independent of system losses.

Absolute reflectance of an unknown material can be determined if oneknows the values of both absolute and measured reflectance of a knownmaterial at the desired wavelength. With this data, the system losses,or system efficiency coefficient, Zλ, at wavelength λ, is computed bymerely dividing the measured value by the absolute value of reflectance.Many reference books list tables of refractive index and absorption ofvarious materials at various .[.frequencies.]. .Iadd.wavelengths.Iaddend.and many also list the values of absolute reflectance atvarious wavelengths. Thus, absolute reflectance values, aR, areavailable or calculable for several pure materials, such as singlecrystal silicon.

With knowledge of an absolute reflectance value, aR, of a particularpure material at some known wavelength λ, the system efficiencycoefficient, Zλ at that wavelength, is determined by measuring themeasured reflectance, mR, and .[.divide.]. .Iadd.dividing .Iaddend.byaR:

    Zλ=mR/aR                                            (1)

To determine the absolute reflectance, aRx, of a material, x, measurethe .[.measured.]. reflectance, mRx, of that unknown material, x, at thesame wavelength, λ, and with the same optical system, and .[.multiply.]..Iadd.divide .Iaddend.the results by the system efficiency coefficient,Zλ.

    .[.aRx=mRx(Zλ).]. .Iadd.aRx=mRx/Zλ.Iaddend.  (2)

The determination of absolute reflectance can readily be performed bythe computer system illustrated in FIG. 1. The value aR of the knownmaterial at the predetermined wavelength is entered through the keyboard28 into the computer 24 which is programmed to perform the simpledivision .[.and multiplication.]. shown in Equations (1) and (2) above.The value, aR is stored in the memory 26. The reflectance, mR is thenmeasured of the known material 18 on the microscopy stage and thedetected value is stored into the memory 26. The computer 24 thenperforms Equation (1) and stores the efficiency coefficient, Zλ inmemory. Without making any changes in the energy source or the opticalsystem, the known material 18 is replaced with the unknown material, x,and the reflectivity is measured to obtain the value, mRx, which isapplied to the computer 24 along with the efficiency coefficient Zλ, inmemory. The computer performs the .[.multiplication.]. .Iadd.division.Iaddend.of Equation (2) to obtain the absolute reflectance, aRx of theunknown material, x.

I claim:
 1. A method for determining an absolute reflectance of materialfrom a microscopic measurement of its measured reflectance in theultraviolet radiation range, said method comprising the stepsof:determining a value of absolute reflectance of a known material at apredetermined wavelength; measuring the reflectance of said knownmaterial to obtain a value of measured reflectance with a microscopeilluminated with radiation at said predetermined wavelength; with saidvalues of absolute reflectance and measured reflectance, calculating anefficiency coefficient representing all absorption and losses caused bythe microscope optical system, its reflectance detectors and itsillumination system at said predetermined wavelength; measuring thereflectance of an unknown material to obtain a second value of measuredreflectance with said microscope illuminated with said radiation at saidpredetermined wavelength; applying said efficiency coefficient to saidsecond value of measured reflectance to obtain a value of absolutereflectance of said unknown material.
 2. The method claimed in claim 1wherein said step of applying includes the step of .[.multiplying.]..Iadd.dividing .Iaddend.said second value by said efficiencycoefficient.
 3. The method claimed in claim 2 wherein said microscope isa reflecting microscope.
 4. The method claimed in claim 3 wherein saidpredetermined wavelength is in the ultraviolet radiation range.
 5. Themethod claimed in claim 2 wherein the determined values of absolutereflectance of said known material, said value of measured reflectanceof said known material and said value of measured reflectance of saidunknown material are stored in a memory of a computer that performs thestep of calculating said efficiency coefficient and said value ofabsolute reflectance of said unknown material.